A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which may be alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to image ever smaller features, it has been proposed to use EUV radiation, with a wavelength in the range of from 5 to 20 nm, as the exposure radiation instead of UV, with a wavelength of 193 or 157 nm, as is used often used in current commercial lithography devices. No material capable of forming a refractive lens for optical elements in radiation and/or projection systems for EUV radiation is known, so the radiation and projection systems of an EUV lithographic apparatus are presently made using optical elements, more specifically mirrors, usually multilayer optical elements. The quality of the projected image is extremely sensitive to surface deformations (figure errors) in the optical elements, particularly those of the projection system.
It has been found that the radiation in lithographic apparatus is partly absorbed by the optical elements. This causes the optical elements to heat up. In particular, it has been found that the EUV projection beam is absorbed by optical elements in the radiation and the projection systems. A further problem is that direct thermal conditioning of the optical elements is not possible due to the high dynamic requirements of the optical elements. In the future, if higher outputs are to be achieved using lithographic apparatus operating at relatively short wavelengths, active conditioning of the optical elements, in particular, the optical elements, in the projection and radiation systems may be required if the optical specifications are to be met even when the optical elements are constructed of materials having a very low coefficient of thermal expansion (“CTE”), for example, materials such as ZERODUR®. Another problem is that lithographic apparatus operating at EUV wavelengths operate in vacuum conditions. EP 1 376 239 (US 2004/051984) is concerned with a cooling device for an optical element, wherein the optical element is indirectly cooled by a heat receiving plate. It has been found that cooling of the optical elements in such a manner is difficult to achieve because of the poor heat transfer mechanism in vacuum, since only radiation cooling is possible. In particular, cooling in a vacuum without direct contact, has been found to be an inefficient heat transfer mechanism. A further problem is that thermal conditioning elements that are disposed in direct contact with the optical elements introduce vibrations which compromise the image quality.